Clutches fall into two types: one which is engaged when subjected to a pressure (and disengaged when no pressure is applied), and the other which is disengaged when subjected to a pressure (and engaged when no pressure acts thereon). The same clutch actuating hydraulic circuit can be used for actuating two types of such clutches by changing the reading of the operation of the hydraulic circuit. In the conventional clutch actuating hydraulic circuit shown in, for example, FIG. 4, if the clutch is of the former type, position 6A of a switch-over valve 6 indicates a clutch engaging position, while position 6B denotes a clutch disengaging position. Conversely, if the clutch is of the latter type, the position 6A indicates the clutch disengaging position, and the position 6B indicates the clutch engaging position. The clutch actuating hydraulic circuit, which will be described below, is one which employs the former type of clutch, unless otherwise indicated.
In FIG. 4, a hydraulic fluid sucked out of a reservoir 1 by a hydraulic pump 2 is applied to a switch-over valve 6 through a pressure circuit 31 as a discharged fluid of the hydraulic pump 2. When the switch-over valve 6 is at the position 6A, the discharged fluid is supplied to a clutch 7 to move a piston 71 and thereby allow a clutch disk and a clutch plate 72 to be pressed against each other, i.e., to engage the clutch 7. When the switch-over valve 6 is switched over to the position 6B, the hydraulic fluid in the pressure circuit 31 cannot advance beyond the position 6B of the switch-over value 6, and the pressure in the pressure circuit 31 thereby rises to the preset value of a pressure limiting valve 5 provided in the pressure circuit 31. Thereafter, the discharged fluid of the hydraulic pump 2 is returned from the pressure limiting valve 5 to the reservoir 1 through a reflux circuit 9. At the same time, the hydraulic fluid in the clutch 7 is also returned from the position 6B to the reservoir 1 through the reflux circuit 9.
However, the above-described conventional clutch actuating hydraulic circuit has the following disadvantages. Generally, discharge of a hydraulic pump required to actuate a hydraulic actuator is determined by a value obtained by multiplying a displacement of the hydraulic actuator by the operating time. Applying this formula to the clutch actuating hydraulic circuit, once the clutch 7 has been engaged, discharge of the hydraulic pump 2 is not required except to compensate for the leakage of the clutch actuating hydraulic circuit. However, in the conventional clutch actuating hydraulic circuit, after the clutch 7 has been engaged, the discharged fluid is pressurized to the preset value of the pressure limiting valve 5, and then returned to the reservoir 1. When the clutch 7 is not operated, i.e., when the hydraulic circuit is closed at the position 6B of the switch-over valve 6, pressure is generated in the clutch operating hydraulic circuit and increased to the preset value of the pressure limiting valve 5, despite the fact that there is no clutch 7 to be operated (discharge of the hydraulic pump 2 is not necessary). After the pressure in the hydraulic circuit has reached the preset value, all the discharged fluid of the hydraulic pump 2 is returned to the reservoir 1. In brief, the driving force for the hydraulic pump 2 to return the fluid to the reservoir is basically unnecessary (output loss).
In view of the aforementioned problems of the conventional technique, an object of the present invention is to provide a clutch actuating hydraulic circuit which is suitable for reduction of output loss.